This invention relates to novel liquid crystal substances which exhibit a positive dielectric anisotropy.
Liquid crystal display elements utilize optical anisotropy and dielectric anisotropy of liquid crystal substances, and are classified into various types such as TN type, DS type, guest-host type, DAP type, White-Tailor type, etc. according to their display modes, and the properties of liquid crystal substances required for their respective uses are different. For example, liquid crystal substances having a positive dielectric anisotropy value .DELTA..epsilon. or those having a negative one may be required, or those having an intermediate value therebetween may be suitable, depending on the kind of the display elements. Liquid crystal substances should exhibit a liquid crystal phase within as broad a temperature range as possible and also be stable to moisture, heat, air, light, etc. At present, however, no single compound alone satisfies such conditions, and it is the present status that several kinds of liquid crystal compounds or non-liquid-crystal compounds have been blended together to obtain practically usable liquid crystal compositions.
Recently, liquid crystal display elements which are actuated within a broader temperature range than previous ones, i.e. within a temperature range from a lower temperature (about -20.degree. C.) to a higher temperature (about 80.degree. C.) have come to be required. Thus, as a constituent of liquid crystals applicable to the above-mentioned use, liquid crystal compounds which exhibit a liquid crystal phase within a broader temperature range and also have a lower viscosity have been needed.
An example of liquid crystal compounds used for such an object is compounds expressed by the general formula ##STR2## wherein R represents an alkyl group and n is 1 or 2. Among these compounds, cyanophenylalkylcyclohexanes (compounds where n=1 in the above formula) disclosed in U.S. Pat. No. 4,130,502 have a N-I point as low as about 50.degree. C.; hence the above object cannot be fully attained. Further, cyanobiphenylalkylcyclohexanes (compounds where n=2 in the above formula) disclosed in Japanese patent application laid-open No. Sho 53-90251/1978 (West German patent application laid-open No. 2,701,591) have N-I points of about 220.degree. C.; hence they are considerably useful for elevating liquid crystal temperature ranges, but have high viscosities of about 65 cp, which restricts the amount of such compounds which can be used. Thus, compounds which exhibit a nematic phase at higher temperatures and yet have a lower viscosity have been required. The object of the present invention is to provide compounds which satisfy such requirements.